As disclosed in Patent Document 1, for example, a partial discharge sensor in a conventional insulation diagnosis apparatus is configured such that outer and inner conductors of signal wires that are connected to a signal extracting port are connected to outer and inner electrodes. In addition, the partial discharge sensor is disposed in a casing connected through an opening; a gap is provided on the inner side of the inner electrode, and a desiccant is inserted into the corresponding part of the gap. With this configuration, a function as the partial discharge sensor and a function of holding the desiccant can be achieved simultaneously. In particular, in a small-sized GIS apparatus having a strict restriction in the size of the whole apparatus, this configuration contributes to downsizing of the whole apparatus.